Patent GB-198755-A (GB)
Title: Improvements in electrical burglar-alarm systems and the like
IPC: G08B13/22
Filed: 1922-03-06
Abstract: 198,755. Broom, J. F. March 6, 1922. Burglar alarms. - In an electrically-operated system, three relays are provided of which the first controls a local alarm circuit and is normally connected through normally closed alarm contacts in series with the second. The third relay controls a switch in the circuit of the first and is operated on the falling back of the second. As shown in Fig. 1, the first relay A has two armatures A<1>, A<2> the latter of which is included in a local alarm circuit and co-operates with a detent D which has conducting and insulating portions and forms the armature of a re-setting magnet D'. Setting. On closing the setting contact SC, for example by shutting the main door, a circuit is completed through contact SP of relay A, for the relays A, B, in series. These relays pull up their armatures and, if the various alarm contacts are set, complete a holding circuit for themselves at contacts CA, BA. Armature B<1> in pulling up co-operates with a cam surface on a pivoted member P<1> so as to complete momentarily the circuit of an indicator, for example a lamp. If on closing switch SC the alarm circuit is not complete, no holding circuit is made and since relay A is self-interrupting at contact SP, armature B<1> of relay B chatters and the indicating lamp flickers. Operation. If, when the alarm is properly set, the lead x<1> is cut or any of the alarm contacts opened, relay A is de-energized and its armature A<2> falls partly back until it engages the conducting end of the member D. A circuit is thus completed from the local batteries LB through the alarm bells S. The first movement of the bells frees the armature of magnets D<2> which fall on to stationary contacts and maintain the ringing circuit independently of relay A. If, instead of cutting lead x<1>, the leads x, x<1> are short-circuited for instance by connecting together two insulated portions of a safe-protecting casing T, the circuit of relay A includes the short-circuit and the armature C' of relay C but relay B is. shortcircuited and armature B' completes at BB a circuit for relay C which pulls up and interrupts the circuit of relay A. The alarm circuit is thereupon completed as in the first case. Resetting. If, the alarm bells having been set in action, the alarm contacts are returned to normal, any short-circuits or disconnections are rectified and the push RP is pushed, a circuit is completed for magnets D<1>, D<2>. Detent D is thus withdrawn from armature A<2> and the armatures of magnets D<2> again engage detents on the bell armatures S<1>. The bells thus stop ringing and the alarm is again set on operation of the contact SC. Contacts. Contact springs C in the local alarm circuit are normally separated by an insulating wedge which is arranged to be withdrawn and cause the alarm to operate if the box containing the bell is tampered with. The contacts W operated by opening a window comprise a conducting roller, Fig. 3, working over a laddershaped conductor. The contacts V operated by breaking a window comprise plates V, Fig. 5, normally held in contact with pins v<1> against a. spring by the pane, but released when the pane is broken. A second pane may be arranged as shown in Fig. 4, with its edges free so that when the panes are broken its upper edge is unsupported and falls, releasing the plates V. Fig. 6 shows a casing made up of panels composed of two layers o' tinfoil separated by waxed paper. The casing is secured by a bar t<1> and wing nuts t<2> which are arranged so that loosening one of them interrupts the alarm circuit. According :o the Provisional Specification, both the armatures of relay A are included in series in the local ringing circuit, and the armature of relay C is arranged, when the leads x, x<1> are short-circuited, to form a vibrating contact so as to weaken the current through relay A sufficiently to allow it to release its armature.